Navigation satellite is an artificial satellite stationed in space for the purposes of navigation. Satellite navigation is a space-based radio positioning system that includes one or more satellite constellations, augmented as necessary to support the intended operation, and that provides 24-hour three-dimensional position, velocity and time information to suitably equipped users anywhere on, or near, the surface of Earth. A satellite navigation system provides users with sufficient accuracy and integrity of information to be useable for critical navigation applications. The GPS system is the first core element of the satellite navigation system widely available to civilian users. The Russian satellite navigation system, GLONASS, which is similar in operation, is another satellite constellation element of GNSS.

The current constellation consists of 21 operational satellites and 3 active spares. Satellites are in orbits with approximately 12-hour periods operating at an altitude of 20,200 kilometres. The orbital constellation consists of six orbital planes, each inclined with respect to the equatorial plane by about 55 degrees. Such an arrangement ensures that at any time there are at least four (and up to 12) satellites above the horizon available for simultaneous measurements. GPS satellites transmit on two L-band frequencies: 1.57542 GHz (L1) and 1.22760 GHz (L2). The L1 signal has a sequence encoded on the carrier frequency by a modulation technique which contains two codes, a precision (P) code and a coarse/acquisition (C/A) code. The L2 carrier contains only P-code that is encrypted for military and authorized civilian users. Most commercially available GPS receivers utilize the L1 signal and the C/A code.

P-code users determine their geocentric positions instantly to about 5 metres with a single hand-held satellite receiver. The C/A codes repeat every millisecond and are available to every user. These codes are also usable for positioning but they provide only about 20- to 30-metre accuracy.

GPS-equipped balloons are monitoring holes in the ozone layer over the Polar Regions, and air quality is being monitored using GNSS receivers. Buoys tracking major oil spills transmit data using GNSS. Archaeologists and explorers are using the system.

Communication

A communications satellite is an artificial satellite stationed in space for the purposes of telecommunications. Modern communications satellites use geosynchronous orbits, Molniya orbits or low Earth orbits.

Geostationary orbits

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The geostationary orbit is useful for communications applications because ground based antennae, which must be directed toward the satellite, can operate effectively without the need for expensive equipment to track the satellite’s motion. Especially for applications that require a large number of ground antennae (such as direct TV distribution), the savings in ground equipment can more than justify the extra cost and onboard complexity of lifting a satellite into the relatively high geostationary orbit.

The first geostationary communications satellite was Anik 1, a Canadian satellite launched in 1972. The United States launched their own geostationary communication satellites afterward, with Western Union launching their Westar 1 satellite in 1974, and RCA Americom (later GE Americom, now SES Americom) launching Satcom 1 in 1975.
By 2000 Hughes Space and Communications (now Boeing Satellite Systems) had built nearly 40 percent of the satellites in service worldwide. Other major satellite manufacturers include Space Systems/Loral, Lockheed Martin (owns former RCA Astro Electronics/GE Astro Space business), Northrop Grumman, Alcatel Space and EADS Astrium.

Molniya Orbits

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Molniya orbits can be an appealing alternative in such cases. The Molniya orbit is highly inclined, guaranteeing good elevation over selected positions during the northern portion of the orbit. (Elevation is the extent of the satellite’s position above the horizon. Thus a satellite at the horizon has zero elevation and a satellite directly overhead has elevation of 90 degrees.

Furthermore, the Molniya orbit is so designed that the satellite spends the great majority of its time over the far northern latitudes, during which its ground footprint moves only slightly. Its period is one half day, so that the satellite is available for operation over the targeted region for eight hours every second revolution. In this way a constellation of three Molniya satellites (plus in-orbit spares) can provide uninterrupted coverage.

Molniya satellites are typically used for telephony and TV services. Another application is to use them for mobile radio systems. The first satellite of Molniya series was launched on April 23, 1965 and was used for experimental transmission of TV signal from Moscow uplink station to downlink stations, located in Russian Far East, in Khabarovsk, Magadan and Vladivostok.

Low-Earth-orbiting satellites (LEO)

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LEO satellites are less expensive to position in space than geostationary satellites and, because of their closer proximity to the ground, require lower signal strength. So there is a trade off between the number of satellites and their cost. In addition, there are important differences in the onboard and ground equipment needed to support the two types of missions.

It is also possible to offer discontinuous coverage using a low Earth orbit satellite capable of storing data received while passing over one part of Earth and transmitting it later while passing over another part.

Applications of communication satellite

Telephony

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The first and still, arguably, most important application for communication satellites is in international telephony. Fixed-point telephones relay calls to an earth station, where they are then transmitted to a geostationary satellite. An analogous path is then followed on the downlink. In contrast, mobile telephones (to and from ships and airplanes) must be directly connected to equipment to uplink the signal to the satellite, as well as being able to ensure satellite pointing in the presence of disturbances, such as waves onboard a ship.

Hand held telephony (cellular phones) used in urban areas do not make use of satellite communications. Instead they have access to a ground based constellation of receiving and retransmitting stations.

Televiosion and Radio

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There are two types of satellites used for television and radio:

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Direct Broadcast Satellite (DBS): DBS is a term used to refer to satellite television broadcasts intended for home reception, also refered to as direct-to-home signals. It covers both analogue and digital television and radio reception, and is often extended to other services provided by modern digital television systems, including video-on-demand and interactive features. A "DBS service" usually refers to either a commercial service, or a group of free channels available from one orbital position targetting one country.

Fixed Service Satellite (FSS): FSS is the official classification for geostationary communications satellites used chiefly for broadcast feeds for television and radio stations and networks, as well as for telephony, data communications, and also for Direct-To-Home (DTH) cable and satellite TV channels. Before the advent of direct broadcast satellite or DBS, technology, FSS satellites were used for DTH satellite TV from the late 1970s into the 1980s, up until the first DBS television system was launched in 1989 for Sky TV in the UK, with DirecTV following suit in the USA in 1994. FSS satellites were the first geosynchronous communications satellites launched in space (such as Intelsat 1 (Early Bird), Syncom 3, Anik 1, Westar 1, Satcom 1 and Ekran).

FSS satellites operate in either the C band (from 3.7 to 4.2 GHz) and the FSS K bands (from 11.45 to 11.7 and 12.5 to 12.75 GHz in Europe, and 11.7 to 12.2 GHZ in the United States).

FSS satellites operate at a lower power than DBS satellites, requiring a much larger dish than a DBS system, usually 3 to 8 feet for K band, and 12 feet on up for C band (compared to 18 to 24 inches for DBS dishes). Also, unlike DBS satellites which use circular polarization on their transponders, FSS satellite transponders use linear polarization.

Systems used to receive television channels and other feeds from FSS satellites are usually referred to as TVRO (Television Receive Only) systems, as well as being referred to as big-dish systems (due to the much larger dish size compared to systems for DBS satellite reception), or, more pejoratively, BUD, or big ugly dish systems.

Mobile Satellite Technology

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Initially available for broadcast to stationary TV receivers, popular mobile direct broadcast applications made their appearance with that arrival of two satellite radio systems : Sirius and XM Satellite Radio Holdings. Some manufacturers have also introduced special antennas for mobile reception of DBS television. Using GPS technology as a reference, these antennas automatically re-aim to the satellite no matter where or how the vehicle (that the antenna is mounted on) is situated. These mobile satellite antennas are popular with some recreational vehicle owners.

Amateur radio

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Amateur operators have access to the OSCAR satellites that have been designed specifically to carry amateur radio traffic. Most such satellites operate as spaceborne repeaters, and are generally accessed by amateurs equipped with UHF or VHF radio equipment and highly directional antennas such as Yagis or dish antennas. Due to the limitations of ground-based amateur equipment, most amateur satellites are launched into fairly low Earth orbits, and are designed to deal with only a limited number of brief contacts at any given time. Some satellites also provide data-forwarding services using the AX.25 or similar protocols.

Satellite Broadband

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In recent years, satellite communication technology has been used as a means to connect to the internet via broadband data connections. This is very useful for users to test who are located in very remote areas, and can't access a wireline broadband or dialup connection.

Weather

Weather forecast use a variety of observations from which to analyses the current state of the atmosphere. Since the launch of the first weather satellite in 1960 global observations have been possible, even in the remotest areas. Observation as obtained from satellite used in Numerical Weather Prediction (NWP) model.

During the 1970s and 1980s a wide range of satellite missions have been launched from which many different meteorological observations could be estimated. Some satellite instruments allowed improved estimation of moisture, cloud and rainfall. Others allowed estimation of wind velocity by tracking features (e.g. clouds) visible in the imagery or surface wind vectors from microwave backscatter.

Satellite imagery (visible, infrared and microwave)

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The most basic form of satellite imagery provides pictures of the current cloud conditions. This is a familiar sight on TV weather forecasts. However, satellite imagery can also undergo various types of quantitative processing to obtain information on important meteorological variables such as wind speed and direction, cloud height, surface temperature, sea ice cover, vegetation cover, precipitation, etc.

The first meteorological satellite was launched in 1960 by the USA and provided cloud cover photography. Originally, satellite images were treated purely as qualitative pictures, which were manually viewed and interpreted by meteorologists. Nowadays though, satellite imagery undergoes a great deal of mathematical manipulation and can yield quantitative analyses of atmospheric temperature, humidity, motion and many more meteorological variables. The major advantage of satellites is their ability to produce near-global coverage, which becomes especially important over oceans and remote, unpopulated land regions, where other methods of observation are impracticable. Over large areas of the southern hemisphere, satellites are the only means of Earth observation. As well as observing changes in surface features such as vegetation and sea surface temperature, satellite imagery can also capture the development of transient features such as clouds of water or ice and plumes of ash or dust.

Two types of satellite having on board instruments used for earth weather images:

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Polar orbiters are positioned about 900 km above the surface of the Earth, in a sunsynchronous orbit, which means they see the same part of the Earth at the same time each day. Polar orbiters make about 14 orbits a day and can view all parts of the atmosphere at least twice a day. Although their temporal resolution is limited, they have high spatial resolution (typically around 1 km between pixels) since they are relatively close to the Earth's surface.

Geostationary satellites are positioned about 36,000 km above the equator in a geostationary orbit, which means they are always fixed in position above one part of the Earth. These satellites scan continuously (hence have high temporal resolution 15-30 minutes), but have limited spatial resolution (typically 3-10 km between pixels).

Radiance is measured by the satellite instrumentation and stored as digital values in two-dimensional arrays of pixels, which make up the image. Different instruments scan at different wavelength bands, and provide different information about the atmosphere:

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Infrared radiation, particularly around 12.5 µm, tells us about the temperature of emitting bodies, such as clouds or the surface in cloud-free regions. IR images are particularly good for viewing clouds and images can be produced at night.

Water vapour radiation, centred around 6.7 µm, measures radiation in the water-vapour absorption band. WV images are good for viewing water vapour distributions in cloud-free areas, and for viewing clouds. Most of the radiation sensed is from the 300-600 hPa layer.

Visible radiation, produced in a wavelength band ~ 0.5-0.9 µm, shows clouds but only by reflected sunlight, so no images are produced at night.

Earth Observation

Understand and analyzing global environmental conditions is an essential element of guaranteeing our safety and quality of life. Among other things, we need to be able to spot environmental disasters in a timely manner, and to monitor and manage the Earth’s natural resources. For this purpose, a number of Earth Observation satellites are in orbit for Earth observations. Data collected by these satellites allow us to understand the processes and interactions among land masses, oceans, and atmosphere. The utility of different data sets for different applications are agriculture, forestry, geology, risk management, cartography, environment, and defence.

Agriculture

Agriculture is one of the most important application fields using Earth Observation data from all missions, where other data sources are often too expensive, or too restricted in scope.Typical applications include crop inventory, yield prediction, soil/crop condition monitoring and subsidy control. The scale of products varies, but typical applications are based on the recognition of individual agricultural parcels.

Geology and related oil, mineral and gas exploration activities make up an application segment that takes full advantage of satellite capabilities. The large-scale satellite view allows the generation of Rock Unit Maps and Tectonic Structure Maps. Interferometry allows the generation of Digital Elevation Models (DEMs) and the monitoring of mining subsidence, while radar data are a powerful tool for off-shore oil seep detection and monitoring. Alternative methodologies, such as the use of existing published maps, ground survey mapping or aerial photography, when available, need be used only when very local and detailed information is required.

Risk management

Risk management is one of the fields where EO data may play a primary role. Three different risk situations may be considered:

Pre-crisis

During crisis

Post-crisis

Products needed in the first situation are mainly related to the collection of land cover, geological and hydrological information, while near-real time mapping and tracking of events is required in crisis and post crisis situations.

Currently satellite data are commonly used for the management of risk situations, but very demanding user requirements (particularly for better revisit times), prevent fully operational use. There are unexploited opportunities in this field.

In the three possible risk management situations, crisis prevention is currently seen as the main opportunity, much more than crisis monitoring and damage assessment. This is mainly due to the fact that the coverage needs of crisis monitoring and damage assessment are less than those required for prevention or for monitoring of an on-going crisis. In addition, the number of crises occurring around the world in one year remains rather small. The importance of post-crisis analysis could be improved if the insurance sector should start operational use of satellite data for the assessment of damage due to natural disasters.

Cartography

Earth Observation data make an excellent basis for medium to large scale cartography. Consequently, this segment makes extensive use of satellite data, especially in those situations where the requirements for accuracy can be met, and alternative data sources are too expensive or even unavailable.
Satellite data, with different processing levels, are used for the generation of cartography and digital elevation models.

For the defence and security, EO information is a key information source, and it is handled with more and more sophisticated Geological Information System instruments. The main applications are the generation of maps, target monitoring and detection, and digital elevation model generation.